Their use spiked when the economy was strong and is slowing now, as it should. Some rorting has been identified but it is far less than Labor and the unions say. Where abuses are found, they should be prosecuted. Fines should be increased.

Preventive measures are also available. Tougher definitions of occupations with skills shortages will make it harder for rorters. So will a crackdown on unscrupulous migration agents, recruitment firms and businesses.

But parts of Labor's package of proposed reforms to 457 visas introduced to Parliament on Thursday will punish employers and, as such, the economy, for no significant reward. What's more, some of the measures won't work.

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With an election in 99 days and a Senate report on the 457 scheme due within three weeks, MPs should resist the temptation for a quick fix. Granted, the public should welcome that part of Labor's bill that is aimed specifically at reducing the small number of rorts.

The legislation forces employers - rather than third parties such as labour hire companies - to sign up directly as sponsors of 457 visa holders. As the Migration Institute of Australia has told the Senate committee: "The use of labour hire firms is problematic as it is often difficult to delineate who is ultimately responsible to ensure that the terms and conditions of the sponsored worker are not breached."

The bill contains ''enforceable undertakings'' that employers will need to accept so they meet market salary rates, specified work conditions and do not allow the on-hiring of a visa holder.

But the public should realise parts of the Labor legislation deliver a very convenient political sop to the union movement. Unions and some in Labor have exaggerated the 457 visa threat to Australian workers, not only to protect the vulnerable but to shore up support for the Gillard government.

True, Fairfax Media has reported examples this week where isolated recruitment firms, errant migration agents and rogue businesses have exploited Filipinos and Indians through the scheme. It is tempting to use that to justify alarm about the system. Predictably, ACTU secretary Dave Oliver has done so.

The labour movement says better enforcement requires extra powers for Fair Work inspectors to enter workplaces to check if a sponsor has broken civil or criminal provisions relating to work conditions. (Remember that unions wanted Labor to expand right to entry more broadly under workplace reforms but the independent MPs have jettisoned that.)

Under the bill, the number of inspectors will rise tenfold, to more than 300. This will force many of the vast majority of compliant employers to constantly justify conditions for their temporary workers, with the likelihood that even minor complaints will end up in tribunals.

The bill mandates the reintroduction of job market testing. Employers will be forced to prove they advertised for Australians first, used job expositions, recruitment firms and/or extra research. This is costly and will not work, as Australian National University legal academics have told the Senate committee: "Businesses who do not want a response to their advertising can manipulate factors to ensure they do not get it.''

And as the Business Council of Australia told the inquiry, employers overwhelmingly choose Australian workers first anyway because it costs so much to access staff through the 457 system.

To the extent that more market testing is required, it should be part of a tougher system of deciding which occupations are placed on the list of skills shortages. University of Adelaide academics suggest the Australian Workforce and Productivity Agency be made responsible for the list to ensure "a more rigorous and targeted approach to identifying Australia's skill needs".

If the bill passes, employers under the 457 scheme will have to spend 1 to 2 per cent of their payroll each year on training for local staff and visa holders for the whole visa period, not only the first year. This is a cost no other employer has to endure.